The nation’s largest teachers union is pulling back on its once-enthusiastic support of the Common Core academic standards, labeling their rollout “completely botched.”

National Education Association President Dennis Van Roekel said he still believes the standards can improve education. But he said they will not succeed without a major “course correction” — including possibly rewriting some of the standards and revising the related tests with teacher input.

Van Roekel’s statement suggests quite a rocky road ahead for the Common Core standards, which are meant to instill more rigorous language-arts and math instruction in public schools — and which have been a priority of the Obama administration.

Forty-five states and the District of Columbia have adopted the standards. Most of those decisions were not controversial. But as new lessons have rolled into classrooms nationwide this year — and as new exams based on the Common Core have been introduced — public outrage has flared, often driven by distrust of the federal role in promoting the standards.

No state has yet pulled out of the standards, but Indiana is close to doing so, and intense debate is expected this spring in several other states, including Michigan, Wisconsin and Georgia.

Though he was careful to say he still supports the standards in concept, Van Roekel’s comments could give opponents of Common Core a boost.

Here’s why: The Common Core has plenty of high-profile backers, including President Barack Obama, former Florida Gov. Jeb Bush and the U.S. Chamber of Commerce.

Yet poll after poll shows that parents trust teachers, above all, to do what’s right by their kids.

Given that trust, the NEA’s support has always been a huge feather in the cap of Common Core supporters. It’s also provided a practical boost: The union has pledged that its members would hold town-hall forums, speak at PTA meetings and do everything they could to persuade a wary public to give Common Core a chance.

Now, however, it’s not clear that teachers can effectively serve as ambassadors. For months, dissident groups of educators, including the Badass Teacher Association, have spoken out against the Common Core. In his open letter, Van Roekel made clear that disillusionment was both widespread and mainstream.

He said 70 percent of teachers believe implementation is going poorly in their schools — and two-thirds report that they have never been asked to give their input on how to introduce the new standards.

“The very people expected to deliver universal access to high quality standards with high quality instruction have not had the opportunity to share their expertise and advice” about how to make it work, he wrote. “Consequently, NEA members have a right to feel frustrated, upset and angry about the poor commitment to implementing the standards correctly.”

Some backers of the Common Core said they didn’t consider the NEA’s new position a blow.

“Ensuring that each state is moving at the right pace given its context is critical to the long-term success of implementation, and it’s essential that educators are involved in the implementation process.” said Chris Minnich, executive director of the Council of Chief State School Officers, which helped write the standards. “Where there are concerns, we look forward to working towards solutions.”

Minnich said he had not seen any survey data showing widespread disillusionment among teachers. Instead, he referred to polls that show a majority of teachers back the Common Core. That support, however, has never been rock-solid. A poll of NEA members taken last spring found that 26 percent supported the standards wholeheartedly, 50 percent backed them tentatively, with reservations, and 13 percent said they didn’t know enough to form an opinion.

Van Roekel’s statement echoed a resolution passed last month by the board of the New York state teachers union, which withdrew support for Common Core as currently implemented.

But Van Roekel represents a far bigger constituency. And until now, the NEA has been among the biggest and most vocal backers of the new academic standards. The Bill & Melinda Gates Foundation, which has spent more than $170 million to develop and promote the Common Core, has given the union millions to support the implementation of the standards. Among other initiatives, the NEA recently rolled out a website featuring thousands of model lessons.

Given the increasing dissent in union ranks, Van Roekel’s statement was not surprising, said Mike Petrilli, executive vice president of the Thomas B. Fordham Institute, which backs the Common Core.

“When the going gets tough, union presidents run for cover,” Petrilli said. He said he still believes that only a minority of teachers oppose the Common Core. “As the head of a democratic organization, Dennis Van Roekel can’t ignore those concerns,” he said. “But here’s hoping that he shows courage, too, in following through on his commitment to higher standards and stronger schools.”

In his open letter, Van Roekel took aim at several policies related to the Common Core.

In particular, he decried the federal policy of requiring all states to test all students in third grade through eighth grade this year. Most versions of the new Common Core tests are not ready for widespread use, so many of the exams children are scheduled to take this spring won’t reflect the new material showing up in classrooms.

“Old tests are being given, but new and different standards are being taught,” Van Roekel wrote. “This is not ‘accountability’ — it’s malpractice.”

The new exams should be ready next spring, but Van Roekel called for caution there, too, demanding a moratorium of at least two years on using new assessments to determine teacher evaluations or school grades. The new tests are meant to be much more difficult than typical standardized tests, with many more open-response questions and essays in addition to fill-in-the-bubble multiple choice. In states that rolled out Common Core tests early, such as New York, students’ scores plummeted.

The nation’s other major teachers union, the American Federation of Teachers, has also expressed concern about the implementation of Common Core and has called for a three-year moratorium on high-stakes testing.

Van Roekel also signaled for the first time that he is open to rewriting some of the standards, which were developed by nonprofit organizations in affiliation with the National Governors Association and Council of Chief State School Officers.

He urged states to work with the NEA and local unions to “review the appropriateness of the standards and recommend any improvements that might be needed.” And he called on them to actively engage teachers in field-testing new Common Core assessments and recommending improvements.

“There’s too much at stake for our children and our country,” Van Roekel told POLITICO, “to risk getting this wrong.”